The Flagstone Bench Formation ranges in age from earliest
Triassic to Norian (Late Triassic)
and is exposed in the Beaver Lake area of the northern Prince Charles
Mountains. This sandstone-dominated
formation rests conformably on the Bainmedart Coal Measures and represents
the
upper part of the
Permian–Triassic Amery Group. It is divisible into three members:
the
Ritchie, Jetty and McKelvey members
(in ascending order). Nine sedimentary facies assignable to three facies
associations (major channel,
crevasse/fan and flood-basin deposits) are recognized within the formation.
Ritchie Member sedimentation
took place during a transition from consistently hygric to seasonally
dry conditions and the unit comprises
sandstone-dominated, sheet-like channel deposits interspersed with few,
thin, mottled, haematite-rich flood-basin
siltstones. Deposition took place under fluctuating discharge conditions
chiefly within the channel
tracts of axially (northwesterly/northeasterly) flowing, low-sinuosity
braided rivers. The Jetty Member
shows a gross upward-fining profile dominated in the lower part by poorly
sorted pebbly sandstones and in
the upper part by ferruginous mudcracked siltstones, mottled palaeosols,
calcrete and thin massive sandstone
sheets. This unit reflects deposition of easterly directed alluvial
fans and extensive flood-basin silt
under a semi-arid climatic regime. The Upper Triassic sandstone-dominated
McKelvey
Member shows a
return to axial drainage along the Lambert Graben with sedimentation occurring
primarily within low-sinuosity
braided channel tracts under wetter climatic conditions.